This invention relates to an optical data transmission apparatus and method. In particular, the invention relates, in a system for transmitting intensity modulated light waves over an optical fiber, to an apparatus and method including a cw laser, a phase modulator connected to the cw laser and an intensity modulator connected to the phase modulator.
Dispersion in optical fibers, which causes pulse broadening and data rate degradation, results from the dependence of propagation delay on optical frequency, as illustrated in FIG. 1.
The upper plot shows schematically that the delay experienced by light propagating in a dispersive optical fiber is close to a linear function of optical frequency. Since the spectrum of a light pulse has a finite width, this dispersion causes pulse broadening. The middle drawing of FIG. 1 shows a single pulse being broadened by about a factor of three by propagation through the fiber, and the lower sketch shows multiple pulses running together so that xe2x80x9conesxe2x80x9d cannot easily be distinguished from xe2x80x9czeroesxe2x80x9d in a bit stream after passing through the fiber.
Dispersion is present in all fibers designed for wavelength-division-multiplex (WDM) communication systems. Although several methods to reduce the effect of dispersion are in use or under development, more effective and less expensive dispersion compensation is still a major need for present digital fiber optic systems. In comparison with the 10 Gb/s systems, which represent the state-of-the-art in commercial service, the deleterious effect of dispersion on the ability to demodulate a transmitted bit stream is magnified by about sixteen times in the 40 Gb/s systems now under development. Prototypes of 40 Gb/s systems produced by companies such as JDS Uniphase and Codeon are presently being evaluated by potential customers, and this new technology is expected to be widely deployed for the first time during 2002. Thus, there is a need in the art for a simple method for on-off modulation of a light beam so that short pulses are produced after propagation through a dispersive fiber medium.
Accordingly, the optical data transmission apparatus and method of the present invention includes, in a system for transmitting intensity modulated light waves over an optical fiber, a cw laser. A phase modulator is connected to the cw laser and an intensity modulator is connected to the phase modulator.
In another embodiment of the invention, in a system for transmitting intensity modulated light waves over an optical fiber, an optical data transmission apparatus includes a cw laser conformed to emit light at substantially a single frequency. A phase modulator is connected in series with the cw laser and is conformed to cause the phase of the light from the cw laser to vary in substantially a quadratic manner as a function of time during a time interval T. An intensity modulator is connected in series with the phase modulator, wherein the intensity modulator is conformed to transmit or block the light from the phase modulator in accordance with an intensity modulation scheme for transmitting binary data such that the transmitted light consists of pulses of temporal width T during which the phase of the light varies in substantially a quadratic manner as a function of time.
In another aspect of the invention, the phase modulator is conformed to adjust the amplitude of the phase change of the light, subject to the constraint that the phase of the light varies in substantially a quadratic manner as a function of time during the time interval T. In a another aspect of the invention, the intensity modulator is selected from a group including Mach-Zehnder and elector absorption modulators. In a further aspect of the invention, the phase modulator is selected from a group including straight waveguides and slow wave waveguides. In another aspect of the invention, a multiplicity of cw lasers each connected to a phase modulator, each of which, in turn, is connected to an intensity modulator, is provided. An optical coupler is connected to each intensity modulator and an optical amplifier is connected to the optical coupler. In a further aspect of the invention, more than two cw lasers are provided in which the frequencies of the more than two cw lasers are different from one another and the frequencies emitted by the more than two cw lasers are in a progression with substantially equal frequency spacing.
In a further aspect of the invention, four cw lasers are provided with a phase modulator connected to each cw laser. An intensity modulator is connected to each phase modulator and a four to one optical coupler is connected to all four intensity modulators. An optical amplifier is connected to the four to one coupler.
In another embodiment of the invention, in a system for transmitting intensity modulated light waves over an optical fiber, a method of producing pulses of light which are compressed in temporal width by transmission over an optical fiber includes the steps of inputting a light wave to a cw laser. A phase modulator is connected to the cw laser, the phase modulator conformed to transmit a phase-modulated pulse of light adjusted for amplitude and width. An intensity modulator is connected to the phase modulator, the intensity modulator conformed to transmit a phase-modulated pulse of light and to block light outside of the pulse of light of a particular width.
In a another embodiment of the invention, in a system for transmitting intensity modulated light waves over an optical fiber, a method of producing pulses of light which are compressed in temporal width by transmission over an optical fiber includes the steps of inputting light from a cw laser which emits light at substantially a single frequency. A phase modulator is connected in series with the cw laser, where and the phase modulator causes the phase of the light from the cw laser to vary in substantially a quadratic manner as a function of time during a time interval T, such that the amplitude in temporal width T can be adjusted. An intensity modulator is connected in series with the phase modulator, wherein the intensity modulator is adjusted to transmit or block the light from the phase modulator in accordance with an intensity modulation scheme for transmitting binary data, such that the transmitted light consists of pulses of temporal width T during which the phase of the light varies in substantially a quadratic manner as a function of time, and the temporal width T can be adjusted.